Observation of enhanced photocurrent response in M-CuInS2 (M = Au, Ag) heteronanostructures: phase selective synthesis and application

Article abstract of DOI:10.1039/c6nj02439d

We report controlled synthesis of CuInS₂ in wurtzite and zinc blende phases by solution based thermal decomposition of dual precursors [In(acda)₃] (acda = 2-aminocyclopentene-1-dithiocarboxylic acid) and [Cu(PPh₃)₂(acda)] (PPh₃ = triphenylphosphine) in the presence of appropriate surface-active agents. Furthermore, the preparation of M-CuInS₂ (M = Au and Ag) heteronanostructures on both the phases has been achieved successfully by hot injection of respective gold and silver precursor solutions into the reaction mixture. The characterization of both pure and hybrid nanostructures was carried out by X-ray diffraction (XRD), UV-vis spectroscopy, energy dispersive X-ray (EDX) study and transmission electron microscopy (TEM). A detailed photovoltaic study has been performed with both pure materials and the twin structures and their photocurrent and photoresponse behavior have been compared. The study reveals that upon loading Au and Ag, the material exhibits high photocurrent efficiency compared to pure CuInS₂. An appreciable increase in the light to dark current density ratio confirms that these materials can be used in the fabrication of promising photovoltaic devices.

Full text of DOI:10.1039/c6nj02439d

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Observation of enhanced photocurrent response in M-CuInS2 (M = Au,
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Ag) heteronanostructures: phase selective synthesis and application †
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Abhisek Brata Ghosh,a Namrata Saha,a Arpita Sarkar,a Amit Kumar Dutta,a
Swarup Kumar Maji,a and Bibhutosh Adhikary*a
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aDepartment of Chemistry, Indian Institute of Engineering Science and Technology,
Shibpur, Howrah 711 103, West Bengal, India
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*Corresponding author Tel: +91ꢀ3326684561 Ext. 512, Fax: +91ꢀ3326682916,
Eꢀmail: bibhutoshadhikary@yahoo.in
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†Electronic supplementary information (ESI) available: Fig S1 to S7 and Table S1 to S6.
CCDC reference numbers 940837 and 954069. For ESI and crystallographic data in CIF
format See DOI: 10.1039/ ………
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The crystallographic data can also be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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28 Abstract
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We report controlled synthesis of CuInS2 in wurtzite and zinc blende phases by solution
based thermal decomposition of dual precursors [In(acda)3] (acda
=
2ꢀ
=
aminocyclopenteneꢀ1ꢀdithiocarboxylic acid) and [Cu(PPh3)2(acda)] (PPh3
triphenylphosphine) in the presence of appropriate surfaceꢀactive agents. Furthermore,
the preparation of MꢀCuInS2 (M = Au and Ag) heteronanostructures on both the phases
has been achieved successfully by hot injection of respective gold and silver precursor
solution into the reaction mixture. The characterization of both pure and hybrid
nanostructures were carried out by Xꢀray diffraction (XRD), Uvꢀvis spectroscopy,
energy dispersive Xꢀray study (EDX) and transmission electron microscopy (TEM).
A detailed photovoltaic study has been performed with both pure materials and the
twin structures and their photocurrent and photoresponse behavior have been
compared. The study reveals that upon loading Au and Ag, the material exhibits high
photocurrent efficiency compared to pure CuInS2. Appreciable increase in current density
ratio of light to dark confirms that these materials can be used in the fabrication of
promising photovoltaic devices.
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52 Introduction
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The decline of fossil fuel reserves along with simultaneous increase of global energy
consumption set a clear threat of energy crisis to the world. Consequently, much effort
has been focused to develop new functional materials with effective photoꢀinduced
carrier transport ability for using in inorganic photovoltaic cells.1,2 In the last few years,
nanoꢀscaled ternary and multinary semiconductor sulfides of suitable bandꢀgap have
drawn considerable attention for the fabrication of low cost high efficiency photovoltaic
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devices.3ꢀ5 Among them semiconductors of I
̶ III ̶ VI2 class such as CuInS2, CuInSe2,
CuGaS2, CuInxGa1ꢀxSe2 have emerged as promising candidates for photovoltaic study as
their band gaps (1.4 ꢀ 2.2 eV) match well with the solar spectrum.6ꢀ11
In recent times, several successful colloidal synthetic methodologies have been
developed for the synthesis of CuInS2 of tunable size and shape.12ꢀ20 In particular
synthesis based on solvothermal decomposition of suitable singleꢀsource or dualꢀsource
precursor complexes have turned out to be highly efficient.21ꢀ30 The synthesis of CuInS2
by the thermal decomposition of sulfur bonded metal complex was first initiated by Hirpo
et al. using (PPh3)2CuIn(SEt)4.31 Later, the method was popularized by Vittal using
thiocarboxylate complexes of Cu(I) and In(III).23,26 Then after few years, Lu and his
coworkers have used dithiocarabamate complexes of In and Cu as dual source precursor
to synthesize phase selective CuInS2.25
More recently instead of pure semiconductor, metalꢀsemiconductor
heteronanostructures have rapidly gained wide interest due to their exciting functional
features.32ꢀ39 This type of coupled materials strongly influences the electron transport
property of the semiconductor and thereby improve their optoelectronic and photovoltaic
performances.40ꢀ45 Significant attempts have been made to synthesize a heterostructure
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with the semiconductors by coupling them with metallic silver, gold, palladium or
platinum. For example, studies have been made with hybrid materials like AuꢀCu2S, Auꢀ
CdS, AuꢀCdSe, AuꢀSnS, AuꢀCu2ZnSnS4, PtꢀCuInS2, AgꢀAgInS2 etc.46ꢀ54 However; in all
these cases the core issue that remained was to establish a proper synthetic strategy for
the formation of heteronanostructure.
In the present work, we report a facile synthetic protocol for the formation of Auꢀ
CuInS2 and AgꢀCuInS2 heterostructures. CuInS2 in two different crystallographic phases
viz wurtzite and zinc blende has been synthesized by solvothermal decomposition of
dithiocarboxylate complexes of indium and copper followed by inꢀsitu coupling of
metallic Au and Ag by hotꢀinjection technique. The crystallographic study of precursor
complexes as well as the phase orientation and the microscopic analysis of the pure and
the hybrid materials have been studied in detail. Finally, photoꢀinduced carrier transport
properties and photocurrent behaviors of both pure and coupled materials have been
investigated systematically.
91 Experimental
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Materials
All chemicals obtained from commercial sources were used without further purification.
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Ammonia, cyclopentanone, carbon disulfide, Cu(NO3)2.3H2O, triphenylphosphine
(PPh3) were purchased from Spectrochem Pvt. Ltd. (India). Indium (III) chloride ( 98%),
gold (III) chloride hydrate (99.9%), silver (I) acetate (>99.0%), oleylamine (OAm,
tech., 70%), 1ꢀdodecanethiol (1ꢀDDT, 98%), oleic acid (OA, tech., 90%),
trioctylphosphine oxide (TOPO, 99%), 1ꢀoctadecene (ODE, tech., 90%) and indium tin
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oxide (ITO) coated glass slides were purchased from Sigma–Aldrich. Solvents were used
as received.
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Synthesis
The ligand 2ꢀaminocyclopenteneꢀ1ꢀdithiocarboxylic acid (Hacda) and the starting
material CuI(PPh3)2 NO3 were prepared according to the literature method.55,56
Synthesis of precursor complexes
Preparation of In(acda)3 (1)
To a methanol solution (10 mL) of InCl3 (0.221 g, 1 mmol), a solution of Hacda (0.475
g, 3 mmol) in the same solvent (10 mL) was added in dropwise manner. Yellow
precipitate appeared immediately. The stirring was continued for 30 min. The
precipitate was collected by filtration, washed thoroughly with methanol, diethyl
ether and dried in vacuum. X–ray quality single crystals were obtained by
recrystallising the solid from acetonitrile at room temperature. Yield 0.46 g (78%).
Anal. Calcd for C18H24InN3S6: C, 36.64; H, 4.07; N, 7.13. Found: C, 36.42; H, 3.94; N,
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7.03 %. IR data (KBr pellet, cm–1): 3286 (s,br), 3062 (w,br), 2900 (m), 1615 (s), 1474 (s),
1450 (s), 1410 (m), 1318 (m), 1288 (m), 1213 (m), 1041 (m), 971 (m), 876 (s), 776 (w),
610 (s) cm–1. UV–Vis [in DMF, λmax, nm ( / M–1 cm–1)] 325 (23700), 398 (126900).
ε
Preparation of Cu(PPh3)2(acda) (2)
[Cu(PPh3)2NO3] (1.95 g, 3 mmol) was dissolved in 15 mL of dichloromethane. To
this a methanol (10 mL) solution of Hacda (0.475 g, 3 mmol) was added in a stirring
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condition. The solution became wine red and stirring was continued for 30 min
during which a deep orange microcrystalline product was deposited. The compound
was filtered, washed with methanol and diethyl ether and dried in air. The single
crystals suitable for Xꢀray analysis were collected from 2:1 acetonitrile ꢀ dichloromethane
solution at room temperature. Yield 1.39 g (62%). Anal. Calcd for C42CuH38NP2S2: C,
67.56 ; H, 5.09; N,1.87. Found: C, 67.25 ; H, 5.16; N, 1.96. IR data (KBr pellet, cmꢀ1):
3445 (m, br), 3055 (m, br), 1599 (s), 1512 (m), 1426 (s), 1305 (m), 1083 (m), 1023 (m),
989 (m), 922 (m), 748 (m), 694 (m). UVꢀvis [in acetonitrile, λmax, nm (ε / Mꢀ1 cmꢀ1)] 438
(102110), 315 (65886).
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Synthesis of nanoparticles by precursor decomposition
Wurtzite (Wz) CuInS2 nanostructure
In a typical synthesis, In(acda)3 (0.147 g, 0.25 mmol), Cu(PPh3)2(acda) (0.187 g, 0.25
mmol), 4.0 mL ODE, 1.0 mL OAm and TOPO (1 g, 2.58 mmol) were loaded in a twoꢀ
necked round bottom flask. The solution was heated to 100 °C and degassed for ~20 min.
The temperature was raised to 260 °C under argon flow, kept it for 10 min and then
cooled to 210 °C. At this temperature 2 ml of DDT was swiftly injected into the reaction
vessel. The heating was further continued for 1 h. After cooling to room temperature, the
nanoparticles were precipitated by adding excess amount of dry ethanol. The product was
collected by centrifugation (8000 rpm, 10 min) and dispersed in toluene and again
reprecipitated with ethanol. The process was repeated several times for purification and
then dried in vacuum.
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Fcc Au - Wurtzite (Wz) CuInS2 heterostructure
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Initially, a stock solution of gold was prepared by dissolving HAuCl4 (0.085 g, 0.25
mmol) in 3 ml OAm in a 25 ml twoꢀnecked round bottom flask. The solution was heated
to 80 °C for 10 min under argon flow during which the light orange solution became
colorless. The solution was cooled to room temperature and used as a gold source.
In(acda)3 (0.147 g, 0.25 mmol), Cu(PPh3)2(acda) (0.187 g, 0.25 mmol), 4.0 mL
ODE, 1.0 mL OAm and TOPO (1 g, 2.58 mmol) were taken in a twoꢀnecked round
bottom flask and heated to 100 °C for ~20 min under vacuum. The mixture was heated to
260 °C under argon flow, maintained at this temperature for 15 min and then cooled to
210 °C. At this stage, 2 mL DDT was rapidly injected into the reaction flask and soon
after it the stock solution of gold was also injected. The reaction was continued for
another 1 h. After cooling to room temperature, the product was centrifuged and purified
using toluene as a solvent and ethanol as a nonsolvent.
Fcc Ag - Wurtzite (Wz) CuInS2 heterostructure
The stock solution of silver was prepared by dissolving AgOAc (0.042 g, 0.25 mmol) in 2
mL OAm and then heating the solution upto 120 °C for 10 min at inert condition. The
clear dark brown solution was used as a silver source.
The Ag ꢀ CuInS2 was synthesized in the same way described above for Au ꢀ
CuInS2.
Zinc blende (Zb) CuInS2 nanostructure
In a typical synthesis, In(acda)3 (0.147 g, 0.25 mmol), Cu(PPh3)2(acda) (0.187 g, 0.25
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mmol), 1 mL OA, 1 mL OAm and 2 mL ODE were taken in a twoꢀnecked flask and
heated to 100 °C under vacuum for 20 min. Then the temperature of the stirred mixture
was raised upto 260 °C under argon flow and kept at this condition for 1 h. Then the
mixture was cooled and the obtained product was collected and purified following the
same procedure described in case of wurtzite CuInS2.
Fcc Au – Zinc blende (Zb) CuInS2 heterostructure
In(acda)3 (0.147 g, 0.25 mmol), Cu(PPh3)2(acda) (0.187g, 0.25 mmol), 1 mL OA, 1 mL
OAm and 2 mL ODE were loaded in a twoꢀnecked flask and degassed under vacuum at
100 °C for 20 min. Then the temperature was elevated to 260 °C at inert condition and
previously prepared stock solution of gold was rapidly injected into the mixture. The
temperature was lowered to 210 °C and kept at this condition for 1 h. After cooling the
mixture to room temperature, the product was collected and washed thoroughly. The
washing procedure was same as stated in the above cases.
Fcc Ag – Zinc blende (Zb) CuInS2 heterostructure
The preparation was similar as that of Au ꢀ Zb CuInS2.
Physical mesurements
Powder XRD patterns were obtained on a Philips PW 1140 parallel beam Xꢀray
diffractometer with monochromatic CuKα radiation (λ=1.540598 Å). Surface
morphologies were studied using a JEOL JEMꢀ2100 transmission electron microscope
(TEM) working at 200 kV. Energy dispersed Xꢀray characterizations (EDX) were
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performed in JEOL JSMꢀ7100F. Infrared spectra were obtained in the range of 4000 ꢀ
400 cmꢀ1 as pressed pellets in KBr on JASCO FTꢀIRꢀ460 Plus. Absorption spectra and
were recorded on a JASCO Vꢀ530 UVꢀvis spectrophotometer. Impedance and IꢀV
measurements were carried out on µꢀAUTOLAB III/FRA2 electrochemical workstation.
C, H and N elemental analyses were performed on a PerkinꢀElmer model 2400 analyzer.
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X-ray crystallography
The diffraction experiments were carried out at 120 K on a Bruker–APEX II SMART
CCD diffractometer using graphite–monochromated Mo–Kα radiation (λ = 0.71073 Å).
The data were processed with SAINT, and absorption corrections were made with
SADABS.57 The structures were solved by direct methods, expanded using Fourier
techniques and refined by full–matrix least–squares methods based on F2 using SHELX–
97.58,59 The relevant crystallographic data for both the complexes are given in Table S1.
Photoelectrochemical measurements
The device for photo electrochemical measurement has been fabricated by the dropꢀ
casting method reported by Wang and his group.60 Initially, the ITO coated glass slide
was washed thoroughly using waterꢀacetoneꢀisopropanol. Then the conducting side of an
ITO coated glass was drop coated with a concentrated solution of sample nanoparticles.
The film was then dried in air and annealed at 200 °C for 1 h to use it as a working
electrode. The photovoltaic characteristics and photo current response was measured
using linear sweep voltammetry within a potential range ꢀ1.0 to 1.0 V at a scan rate of
0.05 Vsꢀ1 using a 300 W Xe lamp. The photo sensitivity of the materials was investigated
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by measuring the current in dark and under illumination chronoamperometrically by light
onꢀoff cycle at a steady voltage of ꢀ0.61 V vs Ag/AgCl (0 V vs RHE) in 25 s intervals.
The whole study was performed using Ag/AgCl as a reference and Pt as a counter
electrode and 0.1 M Na2SO4 solution as the electrolyte (pH 7). Electrochemical
impedance measurements were carried out under illumination within the frequency range
100 kHz to 1Hz using a sinusoidal perturbation potential of 10 mV.
Photoelectrochemical reduction of a “sacrificial reagent” methyl viologen
(MVCl2) was performed to identify the reductive splitting of water.61 The experiment was
carried out in a solution containing 0.01 M MV2+ in 0.1 M Na2SO4 (pH 7) using the same
three electrode system where AuꢀWz CuInS2 thin film was used as the photocathode. A
steady potential of ꢀ0.61 V (vs Ag/AgCl) was applied under dark as well as illuminated
condition and the consequent change in the color of the thin film surface was observed.
227 Results and discussion
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Description of crystal structures
The structure of In(acda)3 consists of two independent units which nominally differ in the
InꢀS bond distances and SꢀInꢀS bond angles. A perspective view of one of the In(acda)3
units is shown in Fig. 1a. Indium is bonded with six sulfur atoms of three bidentate acda
ligands leading to a InS6 polyhedron where the donor atoms are located at an average
distance of 2.60 (2) Å from the metal center. Refinement of the structures was slightly
complicated by the fact that the atoms C3 and C16 are both disordered over two positions
sitting above and below the plane of the other three atoms in the fiveꢀmembered rings.
Furthermore, the acda ligand containing S14 and S15 (in another unit of In(acda)3) is
disordered such that the five membered ring has a minor component (of 0.29 occupancy)
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related by a 180° rotation around the C25ꢀC26 bond. Some restraints were used in order
to ensure a smooth refinement. Selected bond lengths and bond angles are listed in Table
S2 and S3 (in ESI†) respectively. The results clearly reveal that in both the units, the
trans angles are much lower than 180°, which indicates that the geometry of the complex
is highly distorted octahedral. The average InꢀS distance and the value of trans SꢀInꢀS
angles obtained here are in good agreement with those observed for
In{S2CN(CH2CH2)2O}3 by Kaim et al.62
The crystal structure of Cu(PPh3)2(acda) (Fig. 1b) is triclinic with space group Pꢀ
1. Cu(I) is bonded with the S atoms of one bidentate acda ligand and the P atoms of two
monodentate triphenylphosphine ligands acquiring a distorted tetrahedral geometry.
Selected bond lengths and bond angles are given in Table S2 and S3 (in ESI†)
respectively. The average Cu(I)ꢀS bond distance is found to be (2.412 Å) larger than that
of Cu(I)ꢀP distance (2.265 Å).
(a)
(b)
Fig. 1 A perspective view of the structures: (a) In(acda)3 (1) and (b) Cu(PPh3)2(acda) (2).
Minor components of the disorder in (1) have been omitted for clarity.
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Synthesis and characterization of nanoparticles
According to the previous reports, several attempts have been made to synthesize CuInS2
till date.63ꢀ65 Most of them were involved in the reaction of different copper and indium
precursors with various sulfur sources.12ꢀ14,19,20 For example, Korgel and his group has
prepared CuInS2 by the reaction of acetylacetonato complexes of In and Cu along with
sulfur powder at high temperature.13 Some other groups have used acetate salt of metals
(Cu and In) and alkylthiol mixture in a high boiling solvent to prepare CuInS2.12,14 In one
of these cases, extra care was taken to remove Cu2S from the surface of CuInS2,
generated as a byproduct during synthesis.14 Recently, Tang et al. reported an interesting
observation where Cu and In precursors were allowed to react with DDT and ODE at
high temperature.20 They obtained all three phases (wurtzite, zinc blende and
chalcopyrite) by the manipulation of anions of Cu and In sources and adjusting their
concentrations during synthesis. They have also synthesized CuInS2 hexagonal nanoplate
of different thickness and composition by varying reaction temperature and time.19 In our
work, as outlined in scheme 1, we have not only made a clear synthetic protocol of
preparing CuInS2 in two phases (Wz and Zb) but also introduced a facile route to
fabricate MꢀCuInS2 (M = Ag, Au) hybrid nanostructure.
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Scheme 1 Scheme of the synthesis of CuInS2 (Wz and Zb) and MꢀCuInS2 (M = Au, Ag)
heteronanostructure.
Both wurtzite and zinc blende nanocrystals have been synthesized by the coꢀ
decomposition of In(acda)3 and Cu(PPh3)2(acda) in presence of appropriate surface active
stabilizers. In overall synthetic process ODE has been used as solvent and OAm has
served as activating agent which significantly lowers the decomposition temperature of
molecular precursor. In our experiment, we observed the surfactantꢀdependent phase
stabilization of CuInS2 where surface active stabilizers play a crucial role in phase
selectivity. Pure wurtzite CuInS2 was obtained when DDT was used as surfactant along
with TOPO during the decomposition of precursors. In our previous study, we explained
the multipurpose use of DDT in nanopaticle synthesis.66 However, in this case DDT is
probably acting as a stabilizer of wurtzite phase.25,26 On the other hand TOPO is acting as
a passivating agent as well as a stabilizer which can readily binds to the surface of the
growing nanoparticles to reduce the surface energy and to protect nanocrystals from
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agglomeration.26 Fig. 2a shows the XRD patterns of the wurtzite phase where the major
diffraction peaks are observed at the 2θ values 26.31°, 27.76°, 29.70°, 38.52°, 46.11°,
50.40°, 54.08°, 54.90°, 55.89° and 64.07°. The relative peak intensities and positions
corroborate well with the powder diffraction data reported for the wurtzite CuInS2 (No.
186) where the space group is P63mc and the lattice constants a = 3.907Å and c =
6.432Å. It is interesting to note that the characteristic peak of the wurtzite pattern at 38.6°
corresponds to the (102) lattice plane. On the other hand when the same precursor
complexes were reacted with OA and OAm solvent mixture instead of DDT and TOPO at
260° C, the zinc blende phase of CuInS2 was obtained.25 As shown in Fig. 2b, all the
diffraction peaks observed at 27.76°, 32.21°, 46.30°, 54.86°, 67.57° and 74.68° for this
product are attributed to (112), (200), (220), (312), (008) and (316) reflection planes of
the zinc blende crystal system (JCPDS No. 75ꢀ0106, space group = Fꢀ43m, a = 5.531Å).
(a)
(b)
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Fig. 2 Powder Xꢀray diffraction pattern of (a) Wz CuInS2 (No. 186), (b) Zb CuInS2
(JCPDS No. 75ꢀ0106).
It has already been established that the coupled structure formation occurs in three
different ways: 1) epitaxial growth of one material on another, 2) recrystallization of one
material on the other and 3) thermal fusion of a particular lattice plane of two previously
formed materials.67,68 In our case, we have adopted the first approach of synthesis
(Scheme 1). The Au and Ag seeds were prepared separately by the reduction of
corresponding metal salts in OAm at inert condition and then rapidly injected into the
reaction system containing the Cu(I) and In(III) precursors. The formation of hybrid
growth was confirmed by the Xꢀray powder diffraction patterns shown in Fig. 3. The
observed patterns ensured that growth of Au and Ag has been taken place on both
wurtzite (Fig. 3a and 3c) and zinc blende (Fig. 3b and 3d) CuInS2 successfully. Apart
from the reflections of corresponding Wz and Zb CuInS2 planes, the additional peaks
obtained at 2θ values of approximately 38.5°, 44.3°, 64.6° and 76.9° can be indexed to
the lattice planes (111), (200), (220) and (311) of fcc Au (JCPDS no. 04ꢀ0784, space
group = Fm3m and cell constant a = 4.078 Å) and Ag (JCPDS no. 03ꢀ0931, space group
= Fm3m and cell constant a = 4.074 Å).
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365
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(a)
(b)
(c)
(d)
Fig. 3 Powder Xꢀray diffraction pattern of (a) fcc Au ꢀ Wz CuInS2, (b) fcc Au ꢀ Zb
CuInS2, (c) fcc Ag ꢀ Wz CuInS2 and (d) fcc Ag ꢀ Zb CuInS2.
The heterostructure formation was further indicated by Uvꢀvis absorption spectra
of hybrid CuInS2. The sharp plasmon resonance peak of pure metallic gold nanoparticle
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appeared at 523 nm (Fig. S1a in ESI†) was significantly broadened and red shifted to 730
nm on coupling with CuInS2. This phenomenon was commonly observed in Au based
heterostructures due to the delocalization of Au plasmon over the coupled
semiconductors.69ꢀ70 The band gaps of the corresponding nanoparticles have been
calculated from the Tauc’s plot.71 CuInS2 of both wurtzite and zinc blende phases
exhibited the band gap of ~ 1.7 eV which was further dropped to ~1.4 eV on formation of
twin structure (Fig. S1b in ESI†).
Morphological investigation and TEM analysis
Fig. 4(aꢀd) shows low and high resolution TEM images of Wz and Zb CuInS2. The Wz
material in Fig. 4a and 4b appears to be quasiꢀspherical of diameter ranging from 18ꢀ27
nm (particle size distribution is shown in Fig. S2 in ESI†). HRTEM image shown in Fig.
4b inset reveals clearly the lattice fringes with spacing d = 0.318 ± 0.002 nm
corresponding to (002) lattice plane of Wz structure. Fig. 4c represents the TEM image of
Zb CuInS2 with triangular shape of length 20 ± 3 nm. The lattice fringe of the material is
shown in Fig. 4c (inset). The calculated d spacing is 0.196 ± 0.004 nm and corresponds to
the (220) reflection plane of Zb phase. The SAED pattern shown in Fig. 4d reveals
crystalline nature of the material and the (112), (200) and (220) planes of Zb phase can be
labeled clearly. The average crystal size of the materials can be estimated on the basis of
DebyeꢀScherrer equation D = 0.94λ/βcosθ, where β is the peak width at half maxima. The
strongest peak at 2θ = 27.76° for the Wz pattern and at 27.88° for the Zb pattern
respectively were used to calculate their grain size, which are nearly 28 and 27 nm
respectively.
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399
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413
414
415
416
417
418
419
420
(a)
(b)
(c)
(d)
(112)
(200)
(220)
Fig. 4 TEM images of (a) Wz CuInS2 (low magnification), (b) Wz CuInS2 (high
magnification) and (c) Zb CuInS2 (low magnification). (d) SAED pattern of Zb CuInS2.
4b inset) HRTEM image of Wz CuInS2, 4c inset) HRTEM image Zb CuInS2.
Fig. 5a shows a wide view of TEM image of AuꢀWz CuInS2 heteronanostructure. Fig.
5a inset represents the TEM image of single heterostructure where Au is attached to the
corner of CuInS2. The interface combination between Au and Wz CuInS2 has been shown
in Fig. 5b which clearly reveals that the epitaxy is formed by the matching of 5d(111) of
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443
Au and 4d(111) Wz CuInS2. The measured interplanar d spacings of 0.301 and 0.234 nm
can be assigned to (101) plane of Wz CuInS2 and (111) plane of cubic Au(0),
respectively. The selected area electron diffraction (SAED) image at the interface area of
AuꢀWz CuInS2 shown in Fig. 5c supports the epitaxial junction formation as both Wz
CuInS2 (101) and Au (111) planes are identified. In contrast, the situation of the epitaxy
formation at the junction of AuꢀZb CuInS2 is quite different. Here the (111) plane of fcc
Au is coupled with a different plane of Zb CuInS2 than observed for Wz CuInS2. Fig. 5d,
e and f display the TEM and HRTEM images of the AuꢀZb CuInS2 heterostructure. Fig.
5e reveals the HRTEM image of single heterostructure where Au nanocrystal is located at
the periphery of the Zb CuInS2. As shown in Fig. 5f, the interface combination of Au and
Zb CuInS2 is observed between 4d(111) of Au and 3d(112) Zb CuInS2.The lattice line
spacing of 0.32 nm and 0.234 nm can be assigned to (112) plane of Zb CuInS2 and (111)
plane of fcc Au, respectively. The corresponding SAED pattern (Fig. S3 in ESI†) also
supports the coexistence of both Zb CuInS2 and gold.
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461
462
463
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466
(a)
(b)
fcc Au
(111)
(c)
Au (111)
d111 = 0.234 nm
5 × d111
Wz CuInS2 (101)
d101 = 0.301 nm
Wz CuInS2
(101)
(e)
(d)
(f)
Zb CuInS2
(112)
fcc Au
(111)
Fig. 5 (a) TEM images of AuꢀWz CuInS2 (low magnification), inset (a) TEM image of a
single AuꢀWz CuInS2 hetrostructure, (b) HRTEM image indicating the interface
combination of Au and Wz CuInS2, (c) SAED pattern of AuꢀWz CuInS2. (d) TEM image
of AuꢀZb CuInS2 (low magnification), (e) Zoomed view of a single AuꢀZb CuInS2
heterostructure and (f) HRTEM image of AuꢀZb CuInS2.
The morphology of the AgꢀCuInS2 heterostructure is depicted in the high
magnification TEM images shown in Fig. S4 (in ESI†). The significant difference
observed in the growth pattern of AgꢀCuInS2 is that like gold, silver is not deposited
along a particular facet of CuInS2. Fig. S4 (in ESI†) reveals that the silver decoration on
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488
489
the surface of both Wz and Zb CuInS2 is random and haphazard compared to that of gold
and therefore it clearly rules out the chance of epitaxy formation. Moreover, when we
tried to analyze the HRTEM images of the AgꢀCuInS2, no epitaxial junction along a
specific facet was found. Hence, from the microscopic analysis it is evident that the
growth of silver on semiconductor is non epitaxial.
To gain a better understanding of the elemental distribution in these nanomaterials
EDX study was performed with the pure Wz CuInS2 and M ꢀ Wz CuInS2 (M = Au, Ag)
heterostructure (Fig. S5 in ESI). The elemental ratio of Cu:In:S in bare CuInS2 has been
determined to be 1:1.07:1.98, which corroborates well with the required stoichiometric
ratio of 1:1:2. In the case of coupled structure, the study clearly indicates the presence of
Au and Ag along with Cu, In and S in Fig. S5b and c (in ESI†) respectively.
Photovoltaic performances
To investigate the electron transfer ability of the materials the electrochemical impedance
measurements have been performed. Fig. S6 (in ESI†) shows typical Nyquist plots of
pure CuInS2, AuꢀCuInS2 and AgꢀCuInS2 of both Wz (Fig. S6a) and Zb (Fig. S6b in ESI†)
phases. A characteristic semicircle has been obtained in all the cases indicating a single
charge transfer process taking place between the working material and electrolyte. Hence,
it has been fitted on the basis of the Randles equivalent circuit model (Fig. S6c in ESI†)
containing a solution resistance (Rs), a charge transfer resistance (RCt) and a constant
phase element (Q).72 In the Nyquist plot, the high frequency limit of the semicircle
represents the solution resistance Rs whereas the low frequency limit indicates the sum of
solution resistance and charge transfer resistance (Rs+RCt). The parameters associated
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with electrochemical impedance measurement are given in Table S4 (in ESI†). From Fig.
S6 (in ESI†), it is clearly evident that in the cases of both phases, the AuꢀCuInS2
heterostructure display the smallest arc radius (RCt) compared to that of AgꢀCuInS2 and
pure CuInS2 indicating fastest charge transport ability of AuꢀCuInS2 over the others. The
data given in Table S4 (ESI†) substantiate it.
(b)
(a)
(c)
(d)
Fig. 6 Current density versus voltage plots for (a) Wz CuInS2, (b) AuꢀWz CuInS2 and (c)
AgꢀWz CuInS2 under dark and under illuminations. (d) Current density versus time plots
during successive on and off cycles of light for Wz CuInS2, AuꢀWz CuInS2 and Agꢀ Wz
CuInS2 at ꢀ0.61 V vs Ag/AgCl.
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535
In Fig. 6, the photovoltaic characteristics and the temporal photocurrent response
property has been shown. The photocurrent generated by the pure and the hybrid
materials was evaluated using linear sweep voltammetry and the choronoamperometric
measurements under chopped light illumination. Fig. 6a, b and c exhibit the current
density (J) versus voltage plots of Wz CuInS2, AuꢀWz CuInS2 and AgꢀWz CuInS2
nanostructures respectively in dark and under light. In all the cases, the current density
under light and the photocurrent gain (Ilight/ Idark) is significantly higher for the coupled
structure than the pure one (Table S5 in ESI†). The photocurrent density value reported
here is also comparable with that of obtained for several CuInS2 based heterstructures
(Table S6 in ESI†). In the case of pure Wz CuInS2 at ꢀ0.61 V bias vs Ag/AgCl (0 V vs
RHE), the value of Ilight/ Idark is 3.4, while for AuꢀWz CuInS2 and for AgꢀWz CuInS2 the
values are 10.1 and 8.4 respectively. The higher photocurrent value of AuꢀCuInS2 than
that of pure CuInS2 and AgꢀCuInS2 is attributed to the effective excitonꢀplasmon coupling
(Fig. 7) taking place in AuꢀCuInS2 which enhances the electron transfer behavior and
hence the photocurrent density.53 To investigate the photostability of the material the
current density versus time measurements have been carried out by consecutive light on
and off mode at 25 s intervals at a constant potential (ꢀ0.61 V vs Ag/AgCl). The plots of
current density versus time obtained for Wz CuInS2, AuꢀWz CuInS2 and AgꢀWz CuInS2
are shown in Fig. 6d. The study reveals that both the heterostructures have greater
stability with no loss of current density over a period of 250 s whereas the pure Wz
CuInS2 shows a steady decrease of current density with time. The result indicates that the
coupling of metallic Au and Ag enhances the photocurrent sensitivity and photostabilty
of the semiconductor material. Similar feature has also been observed for CuInS2, Auꢀ
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555
CuInS2 and AgꢀCuInS2 of the Zb phase (Fig. S7 in ESI†). The superior carrier transport
phenomenon can be explained by the fact that both Ag and Au exhibit metallic character
to some extent in the heterostructures which efficiently restrain the recombination of
photogenerated electrons. This in turn helps easy transportation of charge carriers along
the semiconductor under illumination resulting high photocurrent efficiency.53
e-
e-
CB
coupling
EF
+
h+
h
Au
VB
CuInS2
Fig. 7 Schematic representation of plasmonꢀexciton coupling occurs in AuꢀCuInS2
heterostructure due to epitaxial growth.
In order to investigate the photoelectrochemical reduction performance of the
material, we have performed the reduction of methyl viologen under a certain applied
bias. On illumination, a deep blue color precipitate of methyl viologen cation radical
(MV+•), which is the photoreduced product, was found to deposit on the working
electrode surface (MV2+colorless to MV+•blue) indicating the reductive splitting of water.61
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557
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560 Conclusion
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563
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566
567
568
569
570
571
572
573
574
575
576
577
578
In summary, we report a simple colloidal approach of synthesis of CuInS2 in two phases
(Wz and Zb) from dual precursors In(acda)3 and Cu(PPh3)2(acda). During synthesis the
surface active agents play a major role in the phase selectivity. Furthermore, we have also
developed a simple route to fabricate the AuꢀCuInS2 and AgꢀCuInS2 heteronanostructure
on both wurtzite and zinc blende phases successfully. From HRTEM characterization, we
can conclude that the heterostructure between the metal and the semiconductor has been
formed epitaxially for gold in both the phases to a particular lattice plane. In addition, our
study confirms the generation of a new material with promising applications. The
photocurrent efficiency of both pure and hetero nanomaterials have been investigated and
the corresponding results revealed that the photocurrent sensitivity and photostability
follow the order of AuꢀCuInS2 > AgꢀCuInS2 > CuInS2 for both the phases. This improved
charge transport ability is definitely attributed to the incorporation of metal on the
semiconductor which helps to migrate the semiconductor electrons through the metal and
thus resulting the photocurrent hike. Finally we believe that our findings will not only
open up a broad area of research of metalꢀsemiconductor heterostructure synthesis but
also provide a deep fundamental insight on designing new functional materials essential
for renewable energy conversion and storage.
579
580 Acknowledgements
581
582
583
Authors are thankful to Prof. K. Nag, Department of Inorganic Chemistry, IACS,
Kolkata, India, for helpful discussion. Authors are indebted to Dr. Supriya Dutta,
Assistant Professor, Nistarini College, Purulia, WB, India, for Xꢀray structure analysis.
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588
Authors also acknowledge the support of Rumeli Banerjee during her project work. DST
(India)(scheme no. SB/S1/ICꢀ33/2013) is acknowledged for research funding. The
authors are also grateful to the MHRD (India), the Department of Chemistry and Dr. M.
N. Dastur School of Materials Science and Engineering, IIEST, Shibpur for providing
instrumental facilities.
589
590
591
592
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596
597
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